Re-use the caught exception's error message in the parse error call.

lib/parse.js

@@ -480,13 +480,11 @@ function tokenizer($TEXT, filename, html5_comments) {
             regexp += ch;
         }
         var mods = read_name();
-        var r;
         try {
-          r = new RegExp(regexp, mods);
+          return token("regexp", new RegExp(regexp, mods));
         } catch(e) {
-          parse_error("Invalid regular expression");
+          parse_error(e.message);
         }
-        return token("regexp", r);
     });
 
     function read_operator(prefix) {